In some internal combustion engines for vehicles, there is one type of engine which includes a control means for controlling an air-fuel ratio at the time of engine start-up. Such a control means provides air-fuel ratio control during engine start-up in order to prevent the occurrence of an engine stall, or to obviate a needless reduction in rotational engine speed as well as discharge of exhaust gases containing harmful components.
In addition, in some of the above control means, there is a certain type of control means for executing learning control to store values obtained during control operations in order to permit such learning values to be reflected for the next control operation.
One such prior example of an air-fuel ratio controller is disclosed in published Japanese Laid-Open Patent Application No. 5-133262. This controller is designed to execute control according to a detection signal from an exhaust sensor so as to bring an air-fuel ratio into a target value by a correction amount being added to or subtracted from a reference amount of fuel. The air-fuel ratio controller is further designed to save the aforesaid correction amount as a learning value, and then to provide control such that the learning value is reflected in calculating further correction amounts. The air-fuel ratio controller is characterized by a control means whereby, when such a saved leaving value is found, in response to engine start-up, to be a correction amount which must be reduced by a quantity greater than a predetermined level with reference to the reference amount, then control is executed so as to cause the air-fuel ratio to achieve the target value by the step of reducing the saved learning value according to a drop in the temperature of cooling water inside the engine. The result is improved operability during cold start-up operations.
There has been disclosed another prior example in published Japanese Laid-Open Patent Application No. 5-222973. As ISC (idle speed control) valve control method for an engine as disclosed in this publication is characterized by the steps of: determining whether an air-conditioner switch is on when closed loop control is in the process of being executed and further when an engine is in a stationary state; when the air-conditioner switch is on, renewing an air-conditioner-on-learning value using a feedback correction amount that is determined based on a difference between rotational engine speed and idle target rotational speed, the air-conditioner-on-learning value being stored at a predetermined address in a storage means, and when the air-conditioner switch is off, then renewing an air-conditioner-off-learning value by means of the feedback correction amount, the learning value being saved at another predetermined address in the storage means; setting an air-conditioner-on-time-learning value as an initial value of the feedback correction amount when the air-conditioner switch is on immediately after start-up-time control is changed to usual time control, and setting an air-conditioner-off-time-learning value as an initial value of the feedback correction amount when the air-conditioner switch is off; and, correcting a basic characteristic value by means of the feedback correction value, the basic characteristic value being determined based on at least engine temperature, and then setting an opening degree of an ISC valve, the ISC valve being disposed at a location along an air bypass passage, the air bypass passage bypassing a throttle valve. As a result, smooth and successful control switching with improved controllability are attained.
There has been disclosed a further example in published Japanese Laid-Open Patent Application No. 6-249019. An idle controller as taught in this publication includes an idle air quantity-regulating means which is capable of regulating intake air quantity for an internal combustion engine in a non-operated state of an accelerator. As a result, an initial value of the aforesaid intake air quantity immediately after engine start-up is higher than an intake air quantity that is obtained when engine warm-up is completed. Then, after the engine start-up, the idle air quantity-regulating means is controlled so as to reduce the intake air quantity in stages. The idle controller is characterized by: an operation state-detecting means for detecting how the engine is run; a decay degree-determining means whereby it is determined on the basis of an operation state detected by the aforesaid detecting means that an intake air quantity before completion of engine warm-up decays to a higher degree with a greater rise in temperature of the engine; and, an idle air quantity control means for controlling the aforesaid idle air quantity-regulating means according to an intake air quantity that is damped and then determined according to a degree of decay. As a result, the intake air quantity during engine start-up is caused to decay according to a state in which the engine is run.
In conventional air-fuel ratio controllers, an idle-learning region condition for learning control has been established, as illustrated in FIG. 15. More specifically, as can be seen from FIG. 15, the following is established as components of the aforesaid idle-learning region condition: to determine whether an idle switch (IDSW) is on/off; to compare a rotational engine speed (Ne) with a fixed value of 1,000 rpm; and, to determine whether an air-conditioner switch (A/C SW) is on/off. Then, the idle-learning region condition is fulfilled when: the idle switch (IDSW) is on; the engine speed (Ne) is equal to or less than the fixed value of 1,000 rpm; and, the air-conditioner switch (A/C SW) is off.
However, there have been drawbacks that occur with learning control under such a conventional idle-learning region condition:
(1) An engine load during cold start-up in a state of an internal combustion engine being cold is designated as area 300 in FIG. 15. This area overlaps with another area of an engine load (referred to as R/L load) when the vehicle is travelling in a state of the engine being warmed up. PA1 (2) In the case of the above (1), a fuel-learning value obtained during vehicle travel, not during idling, enters a start-up injection pulse as a correction when the engine is started in a cold state. PA1 (3) For a fuel-learning value during vehicle travel and, in particular, a fuel-learning value during vehicle travel in the present system, the learning control is executed, even when a purge valve is on. Such a learning value is corrected so as to dilute fuel when the vehicle runs at an elevated altitude or temperature, and, in particular when gasoline vapor occurs in larger amounts. PA1 (4) In the above cases of (1) through (3) above, the engine is turned off, and is then cooled down in a state of the learning value being corrected to provide diluted gasoline. Accordingly, when the engine is again started, then an air-fuel ratio at the engine start-up is corrected to be a leaner ratio. As a result, the engine speed is reduced, or in some instances an engine stall occurs. (See FIGS. 16 and 17.)
In addition, since such a conventional idle time-learning region condition does not include a vehicle velocity-determining item, then deceleration of the engine causes the idle switch to be turned on, even when the vehicle is travelling.
As a result, deviation in an air-fuel ratio during the engine deceleration is recorded as an air-fuel ratio-learning value during idle operation, and then an idle-learning value is erroneously learned. This causes inconveniences, namely one factor contributing to a variation in an air-fuel ratio after a full explosion, and another factor contributing to an incorrect air-fuel ratio during idle operation.